CSFB is specified today in the 3rd Generation Partnership Project (3GPP) Technical Specification (TS) 23.272 (see, e.g., V9.4.0 of June 2010). In brief, CSFB permits in an Evolved Packet System (EPS) the provisioning of voice and other CS domain services (such as services pertaining to Unstructured Supplementary Service Data, or USSD) by re-use of CS infrastructure when the UE is served by the Evolved UMTS Terrestrial Radio Access Network (E-UTRAN). A CSFB-enabled UE, connected to E-UTRAN, may thus use 2nd or 3rd Generation (2G or 3G) technologies such as GSM Edge RAN (GERAN) or UTRAN to connect to the CS domain.
FIG. 1 illustrates an exemplary scenario in which a CSFB-enabled UE connected to E-UTRAN (via the LTE-Uu interface) uses GERAN (via the Um interface) or UTRAN (via the Uu interface) to connect to the CS domain in a CSFB situation (see section 4 of 3GPP TS 23.272). CSFB signaling is realized via the SGs interface between a Mobility Management Entity (MME) associated with the UE and a Mobile Switching Centre Server (MSC-S). The MSC-S has to be CSFB-enabled, which basically means that the MSC-S will have to be capable of maintaining SGs associations towards the MME for EPS attached UEs. A Serving GPRS Support Node (SGSN) is coupled via the S3 interface to the MME and via the Gs interface to the MSC-S.
The SGs interface to the MSC-S is used for mobility management and paging procedures between EPS and the CS domain. CSFB is triggered by paging in E-UTRAN and leads to a Location Update with respect to GERAN or UTRAN (see FIG. 1). 3GPP TS 23.018 (see, e.g., V9.2.1 of October 2010) describes basic call handling mechanisms. An exemplary call handling mechanism for a terminating call to a so-called B subscriber is shown in FIG. 2 (see section 4.2 of 3GPP TS 23.018) for a roaming UE (also called Mobile Station, or MS). The roaming UE is located in a Visited Public Land Mobile Network (VPLMNB).
As illustrated in FIG. 2, when a Gateway MSC of the called B subscriber (GMSCB, or simply GMSC) receives an ISDN User Part (ISUP) Initial Address Message (IAM) for a UE, it requests routeing information from the associated Home Location Register of the B subscriber (HLRB, or simply HLR). To this end, a Send Routing Info (SRI) message is sent to the HLR. Next, the HLR requests a roaming number (i.e., a Mobile Station Roaming Number, MSRN) from the Visitor Location Register currently associated with the B subscriber (VLRB, or simply VLR). In this regard, a Provide Roaming Number (PRN) message is transmitted to the VLR. In a further step the VLR returns the MSRN in a PRN Ack message to the HLR, which forwards the MSRN to the GMSC in a SRI Ack message. The GMSC uses the MSRN to construct an ISUP IAM, which is sent to the Visited MSC of the B subscriber (VMSCB, or simply VMSC).
Upon receipt of the IAM, the VMSC requests information to handle the incoming call from its associated VLR. If the VLR determines that the incoming call is allowed, it requests the VMSC to page the UE. In a next step, the VMSC pages the UE via a Base Station Subsystem associated with the B subscriber (BSSB) using radio interface signaling. In this context, the VMSC starts a local paging timer. When the UE responds, the VMSC informs the VLR in a Page Ack message thereof and stops the paging timer. Next, the VLR instructs the VMSC to connect the call in a Complete Call message, and the VMSC finally establishes a traffic channel to the UE.
The mechanism for handling a terminating call illustrated in FIG. 2 needs to be modified in certain CSFB scenarios. Specifically, CSFB is only available in case E-UTRAN coverage (as defined by Tracking Areas, or TAs) is overlapped by either GERAN or UTRAN coverage (as defined by, for example, Location Areas, or LAs). A general problem results from the fact that there exists no 1:1 mapping between TAs and LAs.
As shown in FIG. 3, for a terminating call the lacking congruency between TAs and LAs may have the consequence that the UE, when falling back from E-UTRAN to GERAN or UTRAN, may land in a LA that is not controlled by the “old” MSC-S towards which the SGs interface association for the UE has been established by the MME. In such a case the “old” MSC-S will not be able to terminate the call. For this reason, a so-called Roaming Retry (RR) procedure is defined in section 5.2 of 3GPP TS 23.018 to allow that call termination is tried again by the GMSC towards the “new” MSC-S controlling the cell the UE is presently camping on.
The RR procedure suggested in section 5.2 of 3GPP TS 23.018 results in the requirement that all GMSCs have to be upgraded for RR support. Such an upgrade may be difficult for network operators having MSC-S and GMSC from different vendors because in practice every MSC-S may as well play the role of a GMSC, and hence all vendors have to provide RR support and all MSC-Ss have to be upgraded. For large countries such as China hundreds of MSC-Ss thus require an update prior to launching CSFB. The fact that the GMSC is located in the home country of the subscriber while, when roaming, the serving MSC-S is located in a visited network further complicates the situation. In such a case RR support has to be coordinated among different network operators, often across international borders.
All these considerations put the current GSM Association strategy at risk, which is to mandate CSFB as the early roaming solution for Long Term Evolution (LTE) networks.